Field of the Invention
The invention relates to a method for operating a nuclear magnetic flowmeter for determining the flow of a multi-phase medium flowing through a measuring tube, having a pre-magnetization unit for pre-magnetization of the medium, having a magnetic field generator for generating a magnetic field interfusing the medium and having a measuring device, wherein the measuring device includes at least one coil-shaped antenna for generating excitation signals exciting the medium and/or for detecting measuring signals emitted by the medium.
Description of Related Art
Nuclear magnetic flowmeters are designed for determining the flow of a medium flowing through a measuring tube. Here, the medium can contain one phase or several phases. In the case of a single-phase medium, the determination of the flow is carried out by determining the flow velocity of the medium. In addition to the determination of the flow velocity, a determination of the relative portions of the individual phases in the medium is part of the determination of the flow of a multi-phase medium.
A basic requirement of the applicability of nuclear magnetic measuring methods is that the medium or each phase of the medium has atomic nuclei with magnetic moments. If a system of atomic nuclei bearing magnetic moments is located in an external magnetic field having a certain direction, then the magnetic moments of the atomic nuclei are oriented in the external magnetic field. The magnetic moments here adopt a state parallel or antiparallel to the outer magnetic field, wherein the state parallel to the outer magnetic field is occupied with a higher probability, so that a “net magnetization” parallel to the outer field is formed in the system. This “net magnetization” is also called equilibrium magnetization. The magnetization can be deflected out of equilibrium by an external disturbance. As soon as the disturbance is gone, the magnetization strives to return to the equilibrium state, i.e., to relax into its equilibrium state. Both the magnetic moment and the outer magnetic field can be described as vectors. In the relaxation process, the vectors of the magnetic moment precess around the vector of the macroscopic magnetic field. The frequency of precession is the Larmor frequency ωL and is proportional to the magnitude of the magnetic field strength B. The Larmor frequency is calculated according to ωL=γ·B, wherein γ is the gyromagnetic ratio, which is at a maximum for hydrogen nuclei. The gyromagnetic ratio indicates the proportionality factor between the angular momentum or the spin of a particle and the associated magnetic moment.
A further requirement for determining the flow of a flowing medium, in particular for the determination of the portions of individual phases of a multi-phase medium is that the individual phases of the medium are able to be excited to different nuclear magnetic resonances.
Nuclear magnetic flowmeters of the type in discussion here are used especially in the analysis of media extracted from oil sources. The medium then consists essentially of the phases crude oil, natural gas and saltwater. All phases contain hydrogen atom nuclei and thus can be excited to nuclear magnetic resonance.
The signals induced in a coil-shaped antenna after excitation by the precessing magnetic moments of the atomic nuclei are used as dependent variable for characterization of the medium. A requirement for the measurement of a multi-phase medium is, as already described above, that the individual phases of the medium are able to be excited to different nuclear magnetic resonances. The magnitude of the electric signal induced in the coil-shaped antenna by the precessing atomic nuclei of a phase of the medium depends on the number of precessing atomic nuclei per volume element in the phase, thus, i.e., dependent on the density of the phase. In a comparison of the average values of the signal amplitudes per cubic meter of gas, oil and water, it can be determined that the signal from gas can be clearly differentiated from that of oil and water. The strength of the signal can be expressed using the so-called hydrogen index HI. The hydrogen index HI describes the relative portion of hydrogen atoms of a medium in comparison to water. Thus, the hydrogen index for water is HIwater=1. For the indices of oil and gas, HIoil=0.9-1.1 and HIgas=0-0.2 hold true. Accordingly, the magnitude of the induced electric signals for the liquid phases is clearly greater than for the gaseous phase.
The magnitude of the electric signal induced by the precessing atomic nucleus of a phase, however, is not only dependent on the number of precessing atomic nuclei per volume element, but additionally is dependent on the exposure time of the atomic nuclei in the external magnetic field. This can be explained simply in that the magnetization has more time to build up at a longer exposure time.
The medium extracted from oil sources and flowing through the measuring tube of the flowmeter can have different flow characteristics. This means that the individual phases of the medium, as seen over the measuring tube cross-section, can be distributed differently. In particular, it is possible that the medium has a so-called stratified flow. The stratified flow is characterized in that the individual phases of the medium flow through the measuring tube in layers. The gaseous phase of the medium is located, here, in the upper part of the measuring tube, the liquid phases of the medium, i.e., the oil phase and water phase, are located in the lower part of the measuring tube. It is not uncommon that the flow velocities of the individual phases of the medium are not identical. The flow profile then has a maximum flow velocity vmax and a minimum flow velocity vmin. Different phase velocities can lead to a so-called “phase slip”, a faster-flowing phase “passing” a slower-flowing phase. This effect of “phase slip” creates a disadvantage, negatively affecting the flow measurement, as is described in the following:
As a given, there is a multi-phase medium having a gaseous phase and a liquid phase flowing through a measuring tube. The gaseous phase has the flow velocity v1, the liquid phase has the flow velocity v2, wherein v1>v2. Furthermore, the measuring tube is interfused with a magnetic field over a constant section with the length L. The magnetic field has at least one component perpendicular to the direction of flow of the medium. Additionally, the medium has a stratified flow characteristic. As described above, the magnetization formed in each phase of the medium is dependent on the exposure time of the medium or the phase in the magnetic field. The gaseous phase with the flow velocity v1 flowing through the section L interfused with the magnetic field remains in the magnetic field for a duration t1, the liquid phase with the flow velocity v2 flowing through section L interfused with the magnetic field remains in the magnetic field for a duration t2. Since the flow velocity of the gaseous phase is greater than the flow velocity of the liquid phase, the exposure time of the liquid phase in the magnetic field is greater than the exposure time of the gaseous phase. This leads to a greater magnetization being able to build up in the liquid phase than in the gaseous phase. The measuring signal of the liquid phase is thus greater than that of the gaseous phase already due to the exposure time in the magnetic field.
It was already stated above that the strength of the measuring signal is dependent on the density or the hydrogen index of the respective phase. It was thus explained that the measuring signal for the gaseous phase, which has a small hydrogen index, is smaller than the measuring signal of the liquid phase, which has a higher hydrogen index, in particular a hydrogen index near 1.
The two influences mentioned above lead, overall, to the measuring signal of a bulk measurement being dominated by the slower-flowing phase, i.e., the liquid phase. The signal strength can be expressed by
  S  ∝      H    ⁢                  ⁢          I      ⁡              [                  1          -                      exp            ⁡                          (                              -                                  L                                      v                    ⁢                                                                                  ⁢                                          T                      1                                                                                  )                                      ]            wherein HI is the hydrogen index, L is the length of the section interfused with the magnetic field, v is the flow velocity and T1 is the spin-lattice relaxation time.
The liquid phase dominating the measuring signal makes the characterization of the gaseous phase complicated and elaborate, and in methods known from the prior art, often leads to relatively inaccurate results.